Display wont let me output 1680x1050 to it through the Mini DisplayPort.

My LG Flatron L227WTG display won't accept it's full resolution of 1680x1050 when it comes from the mini DisplayPort (VGA). It WILL accept 1440 resolution, but not the 1680. I see no problems with the mini DVI port on my old MacBook.

Is this a problem with my display, cord, or did I just buy the wrong adapter? Would using a DVI input fix this? Or do I have to buy the upscaled HD Mini DisplayPort adapter instead?